This invention relates to RF current and voltage measurement, and more particularly to means for accomplishing such measurements for loads which are remotely located or otherwise not directly accessible to measurement instrumentation.
In radar and other RF radiation systems it often is necessary to monitor the radiated power by measurement of the current or voltage at the antenna, particularly in the case of phased array antenna systems. In such systems the power level and phase at each of the individual antenna elements comprising the array affects the direction and shape of the beam or beams formed thereby, so for purposes of system alignment and performance monitoring there is need for a capability to measure power and phase at each element. This presents problems, however, both because of the difficulty of access to individual antenna elements to accomplish the desired measurement, and because the measurement apparatus itself may perturb the parameter to be measured. These problems derive principally from the fact that in array antennas the antenna elements typically are structurally or otherwise closely integrated with their matching networks and sometimes with the transmission lines of the distribution network through which they are fed, making measurements directly at the individual elements difficult if not impossible.
The prior art includes many approaches to measurement of antenna operating parameters, in a number of which the measurements are made remotely from the load element. In some known phased array systems, for example, an approximation of the radiated vector amplitude at each antenna element is derived by interposing a directional coupler in the feed line to the element matching network, and measuring the forward voltage (V.sub.F) as coupled into one arm of the hybrid and the reflected voltage (V.sub.F) as coupled into another arm. After introduction of an empirically determined factor (C.sub.F) to compensate for line transfer, the radiated vector amplitude (V.sub.A) is assumed to be equal to V.sub.F C.sub.F, and the VSWR equal to: ##EQU1##
From the values of V.sub.A and VSWR thus obtained a value of antenna element current amplitude may be calculated. Obviously, however, the accuracy of the result is critically dependent on the validity of the foregoing assumed equality, and this assumption typically is only approximately correct because of mismatch and mutual coupling effects which perturb the relationships assumed.
The present invention has as a primary purpose the provision of apparatus for RF current and voltage measurements which is not dependent on assumed relationships between input and load parameters, and which provides accurate measurement of load current and voltage without requiring direct access to the load for the purpose of such measurements. The apparatus of the invention is adaptable to use with any guided transmission line system, such as waveguide, coaxial cable, stripline and microstrip, and for its operation requires only a knowledge of the characteristics of the matching network at the system operating frequency.